Humidity sensing element



April 29, 1969 P. E. THOMA 3,440,381

HUMIDITY SENSING ELEMENT Filed Sept. 27. 1967 INVENTOR r/f/ogggy xUnited States Patent 3,440,881 HUMIDITY SENSING ELEMENT Paul E. Thoma,Milwaukee, Wis., assignor to Johnson Service Company, Milwaukee, Wis., acorporation of Wisconsin Filed Sept. 27, 1967, Ser. No. 671,067 Int. Cl.G01n 25/56 US. Cl. 73337.5 12 Claims ABSTRACT OF THE DISCLOSURE caneither be a noncrosslinked compound containing glucoside chains or apartially crosslinked reaction prodnet of a compound containingglucoside chains and a monomer capable of reacting with the hydroxylgroups of the glucoside.

The crosslinked core provides resistance to creep, while the outer layerprovides toughness for the element.

Humidity sensing devices are generally classified as a mechanical typeor an electrical type. The mechanical type of humidity sensing deviceutilizes the dimensional change which occurs in the humidity sensingmaterial when there is a change in relative humidity to either indicatethe relative humidity or to actuate a humidity control system, while theelectrical type utilizes a change in the electrical resistance orcapacitance of the humidity sensing element due to a change in relativehumidity.

The common forms of a naturally occurring humidity sensing element, suchas human hair, wood, gold beaters skin, and animal horn, have certaininherent disadvantages. The elements are extremely fragile andfrequently are damaged in shipment. More important, however, theelements are difficult to produce and this is particularly true of thehorn element, for it requires a very precise operation to cut the hornmaterial into thin layers of uniform thickness. Since most of theconventional elements are naturally occurring, it is difiicult to obtainuniform performance from element-to-element and uniformity is onlyobtainable through very careful calibration. Moreover, the elementsgenerally do not retain their original calibration after long termexposures to extremes of humidity and in many cases require frequentre-calibration.

Another disadvantage of some conventional varying dimension humiditysensing elements is that the elements are difiicult to clean, for theycannot satisfactorily be washed with solvents or detergent solutionswithout adversely affecting their'performance. In view of this, theelements must be replaced after a period of use as they cannot readilybe cleaned.

To overcome the disadvantages of the natural occurring materials, therehave been attempts in the past to use synthetic humidity sensingmaterials. Many of these synthetic materials, however, undergo creepwhen stressed. The stresses develop as the result of film production, oras the result of moisture concentration variations within the film, orwhen tensile force is applied to the element to keep the element taut.The resulting reaction to these internally and externally developedstresses results in creep of the film which causes set point andsensitivity changes.

The present invention is directed to a synthetic, vary- 3,440,881Patented Apr. 29, 1969 ing dimension, humidity sensing element which hasimproved resistance to creep when stressed and has improved chemicalresistance. More specifically, the humidity sensing element of theinvention includes a core or base formed of a crosslinked materialproduced by the reaction of a compound containing glucoside chains and astabilizing monomer capable of reacting with the hydroxyl groups of theglucoside, Bonded to at least one surface of the core is a moisturesensitive, organic layer formed of a material having glucoside chains,such as cellulose or cellulose derivatives. In some cases the materialof the outer layer can be partially crosslinked with a monomer capableof reacting with the hydroxyl groups of the glucoside.

The crosslinked core provides resistance to creep when the element issubjected to stress. The outer layers are sensitive to moisture, and asthey are not fully crosslinked materials, provide toughness for theelement, making the element less brittle than an element composedentirely of a crosslinked material.

The element of the invention has improved thermal stability and chemicalresistance due to the crosslinking and can be washed with commercialsolvents or detergent solutions without danger of destroying theperformance of the element.

The humidity sensing element has a rapid response to humidity conditionsand is not affected by extremes of humidity or temperature. The elementhas very little hysteresis and is substantially more stable thannaturaloccurring humidity sensing elements used in the past.

As the element is a synthetic product, it can be fabricated undercontrolled conditions and therefore requires less calibration fromelement-to-element.

Other objects and advantages will appear in the course of the followingdescription.

The drawings illustrate the best mode presently contemplated of carryingout the invention.

In the drawings:

FIG. 1 is a fragmentary perspective view of the humidity sensing elementof the invention;

FIG. 2 is a perspective view of the humidity sensing element in the formof a loop;

FIG. 3 is a perspective view of another modified form of the inventionin which the element is in the form of a strip;

FIG. 4 is a diagrammatic view showing the use of the element in amechanical-type humidity control system; and

FIG. 5 is a diagrammatic view showing the use of the element in anelectrical-type humidity control system.

FIG. 1 illustrates a humidity sensing element 1 comprising a core orbase 2, and outer layers 3 are integrally bonded to opposite surfaces ofthe core.

The core 2 is a substantially fully crosslinked reaction product formedby the reaction of a compound containing glucoside chains, such as acellulosic material, and a stabilizing monomer capable of reacting withthe hydroxyl groups of the glucoside. For example, the reactant can becellulose or a cellulose ester in which the esterifying acids contain upto 20 carbon atoms and preferably up to 6 carbon atoms. Specificexamples are cellulose triacetate, cellulose butyrate, cellulosepropionate, cellulose succinate, cellulose phthalate or the like.Cellulose nitrate can also be used as well as mixed cellulose esterssuch as cellulose acetate-butyrate and cellulose acetatepropionate.Cellulose ethers in which the etherifying alcohol contains up to 8carbon atoms, such as ethyl cellulose, methyl cellulose,hydroxypropylmethylcellulose, and hydroxybutylmethylcellulose can alsobe employed.

The stabilizing reactant which is crosslinked with theglucoside-containing compound can take the form of 3 monomers or partialpolymers of urea-formaldehyde, phenol-formaldehyde,melamine-formaldehyde, triazineformaldehyde, hexamethoxymethylmelamine,glyoxal, 2- hydroxy-adipaldehyde and the like.

The amount of the stabilizing monomer to be used in conjunction with theglucoside derivative can vary depending on the nature of the monomer. Inthe case of a resin which will crosslink with itself such asurea-formaldehyde, the monomer or partial polymer can vary within widelimits. Any excess of the monomer, over and above that which will reactand crosslink with the glucoside will crosslink with itself. However,the stabilizing monomer should react with at least about 1% of theavailable hydroxyl groups of the glucosides and preferably withsubstantially all the available hydroxyl groups. With a stabilizingmonomer or partial polymer that will not crosslink with itself, such ashexamethoxymethylmelamine, the monomer should be used in astoichiometric amount with the glucoside derivative or cellulosicmaterial, or slightly less than a stoichiometric amount, for any excesswill tend to act as a plasticizer for the core 2 and thereby increasethe creep of the element.

To accelerate the crosslinking reaction, a catalyst is usually added tothe reaction mixture. Any conventional catalyst for the particularmonomers or partial polymers being employed can be used. For example,catalysts to be used with urea-formaldehyde, phenol-formaldehyde andmelamineformaldehyde monomers include trifluoroacetic acid,methanesulfonic acid, monobutyl acid orthophosphate, n-butyl acidphosphate, p-toluenesulfonic acid, and the like.

In addition to the catalyst, it may also be desirable in many instancesto employ a catalyst stabilizer which serves to tie up the catalystuntil the crosslinking reaction is desired to occur. The catalyststabilizers are conventional materials and include epoxide monomers andtriethylamine, 2-dimethylaminoethanol, 2-diethylaminoethanol, and othervolatile organic amines having boiling points below 250 C. The epoxidemonomers can be used as both a catalyst stabilizer and as a reactant inthe crosslinking reaction.

The outer layers 3 are preferably formed of a noncrosslinked compoundcontaining glucoside chains, such as cellulose, a cellulose ester, or acellulose ether. With the use of cellulose esters, the esterifying acidscontain up to 20 carbon atoms and preferably up to 6 carbon atoms.Specific examples of cellulose esters are cellulose triacetate,cellulose butyrate, cellulose propionate, cellulose sucoinate, cellulosephthalate, cellulose acetate-butyrate, cellulose acctate-propionate, andthe like. Cellulose nitrate can also be utilized.

When using cellulose ethers, the etherifying alcohol contains up to 8carbon atoms and specific examples are ethyl cellulose, methylcellulose, hydroxypropylmethylcellulose, hydroxybutylmethylcellulose andthe like.

In some cases, the outer layers 3 can be formed of the partiallycrosslinked reaction product of a compound containing glucoside chainsand a monomer or partial polymer capable of reacting with the hydroxylgroups of the glucoside. If the glucoside-containing compound of theouter layers 3 is partially crosslinked, the stabilizing reactant cantake the form of monomers or partial polymers similar to those describedin connection with the formation of the core 2. The amount of thestabilizing monomer to be used in conjunction with the glucosidederivative can vary, but it is important that only a partialcrosslinking be obtained in order that the outer layers 3 will beflexible and tough. Thus, the amount of monomer to be used should beless than the stoichiometric amount required to completely crosslinkwith the hydroxyl groups of the glucoside. It is preferred that thecrosslinking monomer or partial polymer be used in an amount such thatless than 50% of the hydroxyl groups of the glucoside will be tied up bythe crosslinking reaction.

As discussed with the formation of the core 2, a catalyst is usuallyadded to the crosslinking reaction mixture and in some instances it maybe desirable to also add a catalyst stabilizer such as that previouslydescribed.

The outer layers 3, whether non-crosslinked or partially crosslinked,should have a moisture sensitivity such that the outer layer Will show adimensional increase of at least 1% and preferably a 1%% to 7% with achange from 0% to humidity. These sensitivity values are based on theouter layer dissociated from the core and need be in only one direction.In some cases the core 2 will be less moisture sensitive than the outerlayers 3, while in other instances the core can have substantially thesame moisture sensitivity as the outer layers or greater moisturesensitivity than the outer layers.

The thickness of the core 2 has a definite relation to the thickness ofthe outer layers 3. If a relatively moisture insensitive core is usedand is too thick with respect to the thickness of the outer layers, theouter layers cannot provide the necessary dimensional change underchanges in atmospheric moisture to deform the core. For an elementhaving normal response, the thickness of the core will generally be inthe range of about 0.1 to 5 mils, While the thickness of the outerlayers 3 will be less than about 3 mils and should generally be between10 to 400% of the thickness of the core 2. However, this relationshipcan vary depending on the moisture sensitivity and the modulus ofelasticity of the outer layers 3 and core 2 and the response desired.The optimum thickness ratio of the outer surface layer with respect tothe core 2 is generally arrived at experimentally.

It is preferred that the core 2 and outer layers 3 be coextensive inlength and width. However, in some instances, either the core 2 or theouter layers 3 may project beyond the other member of the element, andthe function of the element will not be altered provided mechanicalclamping of the element in use should be made directly to that area ofthe composite film containing the core and outer layers.

As shown in FIG. 1, the moisture sensitive layers 3 are bonded toopposite surfaces of the core 2, and as the core 2 and layers 3, or insome cases only the outer layers 3, change dimension in accordance withvariations in relative humidity, the element will extend and contractlinearly. However, it is contemplated that the moisture sensitive layers3 need only be applied to one surface of the core 2, in which casevariations in relative humidity will cause the element to bow ordeflect, rather than moving linearly.

The core and outer layers 3 are bonded together throughout theirdimensions and various methods may be employed to provide the bondbetween the members. For example, the outer layers 3 can be applied bycoating the core 2 with a solvent solution of the reactants andsubsequently evaporating the solvents and heating the laminatedstructure to achieve the crosslinking reaction. Alternately, the outerlayers 3 can be bonded to the fully polymerized or crosslinked core 2 byuse of auxiliary adhesives.

The sensitivity of the humidity sensing element can be further increasedby hydrolyzing the outer surface of the cellulosic outer layer 3 toregenerated cellulose. The cellulosic outer layer 3 can be subjected tothe influence of either an alkaline or an acid medium to hydrolyzesubstantially all of the acid radicals in the surface layer to therebyobtain a regenerated cellulose film which provides maximum moisturesensitivity. The hydrolyzation can be accomplished by dipping theelement into an alkaline or acid bath and maintaining it in the bath fora period of time suflicient to hydrolyze the acid groups on the sur faceof the outer layers 3. Alkaline materials which can be employed for thehydrolyzation are aqueous or alcoholic solutions of alkali metal bases,such as sodium hydroxide, potassium hydroxide, or lithium hydroxide. Al-

ternately, alcoholic solutions of strong organic bases, such astetramethylguanidine, trimethylamine, or benzyltrimethylammoniumhydroxide can be used for the hydrolyzing action.

Hot alkaline solutions are preferred to increase the reaction rate. Thetime of contact or immersion in the alkaline solution depends, ofcourse, on the material used, the temperature, and strength of thesolution. For example, a two-hour hydrolysis period, using a 5% sodiumhydroxide solution, was required to hydrolyze a mixed cellulose esterouter layer 3 to obtain the desired high sensitivity. By increasing thestrength of the solution to 50%, an almost immediate hydrolyzationoccurred. The most effective reaction conditions were found to beobtained by immersing the element in 230 F., 40% sodium hydroxidesolution for 1 to 4 minutes, depending on the desired layer thickness ofthe hydrolyzed layer.

After the hydrolyzation, the element is preferably rinsed in water toremove and dilute the alkaline residue.

Solutions of mineral acids, such as hydrochloric acid and sulphuricacid, can also be used to hydrolyze the cellulosic outer layer 3.However, the use of alkaline material provides a faster hydrolyzationand is preferred.

The preferred method of preparing the humidity sensing element is toinitially dissolve the components of one of the outer layers 3 in asolvent, such as acetone, ethyl acetate, ethylmethyl ketone, butylalcohol, methylene chloride, nitroethane, cyclohexanone, ethylenedichloride, methylisobutylketone, isobutylacetate, hexane, toluene,diethyl ether, water, ethyl alcohol, xylene, isopropyl alcohol, or thelike. It is preferred to dissolve the materials in the solvent orsolvent mixture in a closed container with mixing or agitation. Thesolution is then cast onto a glass plate with an adjustable strike-01fbar. After the solvent has evaporated to form the first outer layer 3, asolvent solution of the core material is then cast over the dried firstouter layer by use of the strike-off bar.

The solvent solution of the core material contains the glucosidecompound and the stabilizing monomer along with the catalyst and thecatalyst stabilizer.

When the solvent solution of the core material is cast onto the firstouter humidity sensitive layer, the solvents will partially dissolve thesurface of the previously cast outer layer and when the solvent of thecore solution is evaporated, an adherent bond is provided between thetwo layers.

Following the drying of the core 2, a solvent solution of the secondouter layer is cast over the dried core. Again, the solvent will tend todissolve the surface of the dried core, and when the solvent isevaporated, an adherent bond results between the core 2 and the secondmoisture sensitive layer 3.

After the three-layer film is dry, the core 2, and in some cases themoisture sensitive layers 3, are polymerized or crosslinked by heatingthe laminated film on the glass plate to a temperature in the range of200 to 400 F. and preferably 250 to 375 F. for a period of timesufficient to crosslink the stabilizing monomer with the hydroxyl groupsof the glucoside chains.

While the crosslinking or polymerization reaction can be made to occurat room temperature with most formulations, better results are obtainedwhen the reaction is carried out at an elevated temperature.

Specific examples of solvent solution formulations for the outermoisture sensitive layer 3 and core layer 2 are as follows in weightpercent.

EXAMPLE NO. 1

(A) Outer layer solutionuncrosslinkable:

10.00% cellulose acetate butyrate (17% combined butyryl) 90.00% ethylacetate (B) Crosslinkable core solution:

6.20% cellulose acetate butyrate (26% combined butyryl)urea-formaldehyde (crosslinking material) triethylamine (catalyststabilizer) p-toluenesulfonic acid (catalyst) 2.21% n-butyl alcohol2.06% ethyl alcohol 40.60% diacetone alcohol 41.00% ethyl acetateEXAMPLE NO. 2

(A) Outer layer solution-uncrosslinkable:

10.00% cellulose acetate butyrate (37% combined butyryl) 90.00%methylene chloride (B) Crosslinkable core solution:

5.20% cellulose acetate butyrate (37% combined butyryl) 5.58%urea-formaldehyde (crosslinking material) 1.60% triethylamine (catalyststabilizer) 0.15% p-toluenesulfonic acid (catalyst) 2.01% n-butylalcohol 1.86% ethyl alcohol 33.60% diacetone alcohol 50.00% methylenechloride EXAMPLE NO. 3

Outer layer solution-partially crosslinkable: 10.00% cellulose acetate(39.4% acetyl) 1.00% hexamethoxymethylmelamine (crosslinking material)1.60% 2-.amino-2methyl-l-propanol (catalyst stabilizer) EXAMPLE NO. 4

(A) Outer layer solutionuncrosslinkable:

10.00% ethyl cellulose (45.5 to 46.8% ethoxyl content) 90.00 methylenechloride Crosslinkable core solution:

8.00% ethyl cellulose (45.5 to 46.8% ethoxyl content) 3.00%melamine-formaldehyde (crosslinking material) 1.60%2-dimethylaminoethanol (catalyst stabilizer) 0.15 p-toluenesulfonic acid(catalyst) 1.15% n-butyl alcohol 1.00% xylene 85.10% methylene chlorideA specific example of preparing the humidity sensing element of theinvention is as follows:

The outer layer solvent solution shown in Example No. 1 was cast on aclean glass plate with an adjustable strikeoif bar. After evaporation ofthe solvent, the solvent solution of the core material shown in ExampleNo. 1 was then cast on the dried first outer layer by means of thestrike-off bar. Following the evaporation of this solvent, the outerlayer solution of Example No. 1 was then cast on the dried core with thestrike-off bar to provide the second moisture sensitive layer. Onevaporation of the solvent, a three-layer laminate was produced witheach outer layer having a thickness of about 0.3 mil and the core havinga thickness of about 0.4 mil.

The composite structure, while on the glass plate, was heated to atemperature of 350 F. for 12 minutes to crosslink the cellulose acetatebutyrate and the ureaformaldehyde monomer. The resulting laminate wascut into strips and the strips, when employed as humidity sensingelements, showed high sensitivity to moisture, excellent resistance tocreep when subjected to stress and were resistant to chemical attack.

FIG. 2 shows a form of the invention in which the humidity sensingelement 4, similar in construction to element 1, is in the shape of aloop. In this embodiment, the ends of the looped element 4 are disposedaround rollers or supports 5 and mounting brackets 6 are attached to therollers and serve to connect the rollers to a supporting structure. Inservice, the looped element is subjected to tensile stress, as forexample, by providing a spring-loaded connection between one of thebrackets 6 and the supporting structure. A variation in relativehumidity will cause the element 4 to expand or contract in a linear orlongitudinal direction to thereby produce a condition which can be usedto indicate either the degree of moisture in the atmosphere or toactuate a humidity control device.

FIG. 3 illustrates a second modified form of the invention in which theelement 7, similar in structure to element 1, is in the form of a strip.The ends of the element 7 are secured to clamps 8 which are connected toa supporting structure. As in the case of the element shown in FIG. 2,variations in moisture content in the atmosphere will cause the element7 to expand and contract linearly and the expansion and contraction canbe utilized in a conventional manner to provide an indication of themoisture in the atmosphere.

FIG. 4 is a schematic representation showing a simple, mechanical-typehumidity indicator utilizing the element 7. In this embodiment, one endof the element 7 is permanently anchored to a fixed support 9, while theopposite end of the element is attached to a pointer 10 which ispivotally mounted at 11 and is adapted to move along a scale 12. Thesensing element 7 is held under tension by a spring 13 attached tosupport 14. With this arrangement, the linear expansion and contractionof the element 7 will tend to pivot the pointer 10 and provide relativehumidity readings on the calibrated scale 12.

FIG. 5 illustrates an electrical-type humidity indicator in which oneend of the element 7 is permanently fixed to a support 15 and theopposite end of the element is connected to an indicator 16, and isunder tension by a spring 17 attached to support 18. The indicator 16 ispivotally mounted at 19 and the opposite end of the indicator isprovided with a wiper arm 20 which is adapted to move across a variableresistance element 21 connected in an electrical circuit with an ammeter22 and a source of power 23. Changes in dimension of the element 7 serveto move the wiper arm 20 along the resistance element 21 to vary thecurrent flow and thereby provide an indication of the humidity by thecalibrated ammeter at the location of the sensing element or at a remotelocation. Alternately, the varying current in the circuit produced bychanges in dimension of the element 7 may be used as an input signal tohumidity control equipment.

The humidity sensing element of the invention, when under tensilestress, has improved resistance to creep and will thereby retain its setpoint and sensitivity to humidity changes throughout substantial periodsof service without need for calibration.

The outer layers 3 provide an improved degree of sensitivity, as well astoughness for the element, enabling it to be subjected to substantialtensile stress without fracture or cracking.

Due to the crosslinked structure of the core 2, and in some instancesthe partially crosslinked structure of the moisture sensitive layers 3,the element is more resistant to solvents and detergents thanconventional elements and thus can be washed or cleaned with solvents ordetergent solutions without destroying the performance of the element.

As the element is fabricated synthetically under controlled conditions,the characteristics of the element are more uniform and lesselement-to-element calibration is required.

I claim:

1. A synthetic humidity sensing element, comprising a first layer, and asecond layer extending over a substantial portion of surface of saidfirst layer and bonded thereto, said first layer being the substantiallyfully crosslinked reaction product of a compound containing glucosidechains and a stabilizing monomer or partial polymer capable ofcrosslinking with the hydroxyl group of said glucoside chains, and saidsecond layer being selected from the group consisting of (a) anon-crosslinked compound containing glucoside chains, and (b) apartially crosslinked reaction product of a compound containingglucoside chains and a stabilizing monomer or partial polymer capable ofcrosslinking with the hydroxyl groups of said glucoside chains, saidcrosslinked first layer acting to prevent creep of the element and saidsecond layer providing toughness for the element.

2. The element of claim 1 and including a third layer disposed on theouter surface of said second layer and composed of regeneratedcellulose.

3. The element of claim 1 wherein said compounds are cellulosicmaterials.

4. The element of claim 1 in which said compounds are cellulose estersin which the esterifying acid contains up to 20 carbon atoms.

5. The element of claim 1, in which said monomers are selected from thegroup consisting of urea-formaldehyde, phenol-formaldehyde,melamine-formaldehyde, triazine-formaldehyde andhexamethoxymethylm'elamine.

6. The element of claim 1, in which said compound of said second layeris a cellulose derivative, and said element includes a third layerbonded to the outer surface of said second layer, said third layer beingproduced by the hydrolyzation of said cellulose derivative andconsisting essentially of cellulose.

7. A humidity sensing element, comprising a core being the substantiallyfully crosslinked reaction product of a compound containing glucosidechains and a stabilizing monomer or partial polymer capable ofcrosslinking with the hydroxyl groups of said glucoside chains, andouter moisture sensitive layers bonded to opposite surfaces of saidcore, said outer layers being selected from the group consisting of (a)a non-crosslinked compound containing glucoside chains, and (b) apartially crosslinked reaction product of a compound containingglucoside chains and a stabilizing monomer or partial polymer capable ofcrosslinking with the hydroxyl groups of said glucoside chains.

8. The element of claim 7 wherein said compound is cellulose or acellulose derivative.

9. The element of claim 7 in which the monomer or partial polymer isselected from the group consisting of urea formaldehyde, phenolformaldehyde, melamineformaldehyde, triazine-formaldehyde andhexamethoxymethyl-melamine.

10. In a humidity sensing device, a humidity sensing element comprisinga core being the substantially fully crosslinked reaction product of acompound containing glucoside chains and a stabilizing monomer orpartial polymer capable of crosslinking with the hydroxyl groups of saidglucoside chains, and outer moisture sensitive laye rs bonded toopposite surfaces of said core, said outer layers being selected fromthe group consisting of (a) a non-crosslinked compound containingglucoside chains, and (b) a partially crosslinked reaction product of acompound containing glucoside chains and a stabilizing monomer orpartial polymer capable of crosslinking with the hydroxyl groups of saidglucoside chains, means for con- 9 10 nesting one end of said element toa fixed object, and References Cited lsgtiaczlmeslefiznzttpplyingtensile stress to the opposite end of UNITED STATES PATENTS 11. Theelement of claim 1, wherein said second layer 2,604,423 7/1952sloiterbeck et 73337 X shows a dimensional increase greater than 1% witha 5 3,295,088 12/1966 smfth X change in relative humidity from 0% to100%. 330L057 1/1967 Smlth et a1 12. The element of claim 1, wherein thefirst layer has a thickness of 0.1 to 5 mils and the second layer has aLOUIS PRINCE Pnmary Exammer thickness of 10% to 400% of the first layer.JOSEPH W. ROSKOS, Assistant Examiner.

